Optoelectronic devices rely on the optical and electronic properties of materials to either produce or detect electromagnetic radiation electronically or to generate electricity from ambient electromagnetic radiation. Photosensitive optoelectronic devices convert electromagnetic radiation into electricity. Solar cells, also known as photovoltaic (PV) devices, are specifically used to generate electrical power. PV devices are used to drive power consuming loads to provide, for example, lighting, heating, or to operate electronic equipment such as computers or remote monitoring or communications equipment. These power generation applications also often involve the charging of batteries or other energy storage devices so that equipment operation may continue when direct illumination from the sun or other ambient light sources is not available. As used herein the term "resistive load" refers to any power consuming or storing device, equipment or system. Another type of photosensitive optoelectronic device is a photoconductor cell. In this function, signal detection circuitry monitors the resistance of the device to detect changes due to the absorption of light. Another type of photosensitive optoelectronic device is a photodetector. In operation a photodetector has a voltage applied and a current detecting circuit measures the current generated when the photodetector is exposed to electromagnetic radiation. A detecting circuit as described herein is capable of providing a bias voltage to a photodetector and measuring the electronic response of the photodetector to ambient electromagnetic radiation. These three classes of photosensitive optoelectronic devices may be characterized according to whether a rectifying junction as defined below is present and also according to whether the device is operated with an external applied voltage, also known as a bias or bias voltage. A photoconductor cell does not have a rectifying junction and is normally operated with a bias. A PV device has at least one rectifying junction and is operated with no bias. A photodetector has at least one rectifying junction and is usually but not always operated with a bias.
Traditionally, photosensitive optoelectronic devices have been constructed of a number of inorganic semiconductors, e.g. crystalline, polycrystalline and amorphous silicon, gallium arsenide, cadmium telluride and others. Herein the term "semiconductor" denotes materials which can conduct electricity when charge carriers are induced by thermal or electromagnetic excitation. The term "photoconductive" generally relates to the process in which electromagnetic radiant energy is absorbed and thereby converted to excitation energy of electric charge carriers so that the carriers can conduct, i.e., transport, electric charge in a material. The terms "photoconductor" and "photoconductive material" are used herein to refer to semiconductor materials which are chosen for their property of absorbing electromagnetic radiation of selected spectral energies to generate electric charge carriers. Solar cells are characterized by the efficiency with which they can convert incident solar power to useful electric power. Devices utilizing crystalline or amorphous silicon dominate commercial applications and some have achieved efficiencies of 23% or greater. However, efficient crystalline-based devices, especially of large surface area, are difficult and expensive to produce due to the problems inherent in producing large crystals without significant efficiency-degrading defects. On the other hand, high efficiency amorphous silicon devices still suffer from problems with stability. More recent efforts have focused on the use of organic photovoltaic cells to achieve acceptable photovoltaic conversion efficiencies with economical production costs.
PV devices typically have the property that when they are connected across a load and are irradiated by light they produce a photogenerated voltage. When irradiated without any external electronic load, a PV device generates its maximum possible voltage, V open-circuit, or V.sub.OC. If a PV device is irradiated with its electrical contacts shorted, a maximum short-circuit current, or I.sub.SC, is produced. When actually used to generate power, a PV device is connected to a finite resistive load and the power output is given by the current voltage product, I.times.V. The maximum total power generated by a PV device is inherently incapable of exceeding the product, I.sub.SC.times.V.sub.OC. When the load value is optimized for maximum power extraction, the current and voltage have values, I.sub.max and V.sub.max respectively. A figure of merit for solar cells is the fill factor ff defined as: ##EQU1##
where ff is always less than 1 since in actual use I.sub.SC and V.sub.OC are never obtained simultaneously. Nonetheless, as ff approaches 1, the device is more efficient.
When electromagnetic radiation of an appropriate energy is incident upon a semiconductive organic material, for example, an organic molecular crystal (OMC) material, or a polymer, a photon can be absorbed to produce an excited molecular state. This is represented symbolically as S.sub.0 +hv.fwdarw.S.sub.0 *. Here S.sub.0 and S.sub.0 * denote ground and excited molecular states, respectively. This energy absorption is associated with the promotion of an electron from a bound state in the valence band, which may be a .pi.-bond, to the conduction band, which may be a .pi.*-bond, or equivalently, the promotion of a hole from the conduction band to the valence band. In organic thin-film photoconductors, the generated molecular state is generally believed to be an exciton, i.e., an electron-hole pair in a bound state which is transported as a quasi-particle. The excitons can have an appreciable life-time before geminate recombination, which refers to the process of the original electron and hole recombining with each other as opposed to recombination with holes or electrons from other pairs. To produce a photocurrent the electron-hole pair must become separated. If the charges do not separate, they can recombine in a geminate recombination process, either radiatively--re-emitting light of a lower than incident light energy--, or non-radiatively--with the production of heat.
Either of these outcomes is undesirable in a photosensitive optoelectronic device. While exciton ionization, or dissociation, is not completely understood, it is generally believed to occur in regions of electric field occurring at defects, impurities, contacts, interfaces or other inhomogeneities. Frequently, the ionization occurs in the electric field induced around a crystal defect, denoted, M. This reaction is denoted S.sub.0 *+M.fwdarw.e.sup.- +h.sup.+. If the ionization occurs at a random defect in a region of material without an overall electric field, the generated electron-hole pair will likely recombine. To achieve a useful photocurrent, the electron and hole must be collected separately at respective opposing electrodes, which are frequently referred to as contacts. This is achieved with the presence of an electric field in the region occupied by the carriers. In power generation devices, i.e., PV devices, this is preferably achieved with the use of internally produced electric fields that separate the generated photocarriers. In other photosensitive optoelectronic devices, the electric field may be generated by an external bias, e.g., in a photoconductor cell, or as a result of the superposition of internally and externally generated electric fields, e.g., in a photodetector. In an organic PV device, as in other photosensitive optoelectronic devices, it is desirable to separate as many of the photogenerated electron-hole pairs, or excitons, as possible. The built-in electric field serves to dissociate the excitons to produce a photocurrent.
FIG. 1 schematically depicts the photoconductive process in organic semiconducting materials. Step 101 shows electromagnetic radiation incident upon sample of photoconductive material between two electrodes a and b. In step 102, a photon is absorbed to generate an exciton, i.e., electron-hole pair, in the bulk. The solid circle schematically represents an electron while the open circle schematically represents a hole. The curving lines between the hole and electron are an artistic indication that the electron and hole are in an excitonic bound state. In step 103, the exciton diffuses within the bulk photoconductive material as indicated by the exciton's closer proximity to electrode a. The exciton may suffer recombination in the bulk material away from any field associated with a contact or junction as indicated in step 104. If this occurs the absorbed photon does not contribute to the photocurrent. Preferably the exciton ionizes within the field associated with a contact or junction as indicated by the progression from step 103 to step 105. However, it is still possible for the newly liberated carriers to recombine as indicated in step 106 before permanently separating and contributing to the photocurrent. Preferably the carriers separate and respond to the field near a contact or junction according to the sign of their electric charge as indicated by the progression from step 105 to step 107. That is, in an electric field, indicated by e in step 107, holes and electrons move in opposite directions.
To produce internally generated electric fields which occupy a substantial volume, the usual method is to juxtapose two layers of material with appropriately selected conductive properties, especially with respect to their distribution of molecular quantum energy states. The interface of these two materials is called a photovoltaic heterojunction. In traditional semiconductor theory, materials for forming PV heterojunctions have been denoted as generally being of either n, or donor, type or p, or acceptor, type. Here n-type denotes that the majority carrier type is the electron. This could be viewed as the material having many electrons in relatively free energy states. The p-type denotes that the majority carrier type is the hole. Such material has many holes in relatively free energy states. The type of the background, i.e., not photogenerated, majority carrier concentration depends primarily on unintentional doping by defects or impurities. The type and concentration of impurities determine the value of the Fermi energy, or level, within the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), called the HOMO-LUMO gap. The Fermi energy characterizes the statistical occupation of molecular quantum energy states denoted by the value of energy for which the probability of occupation is equal to 1/2. A Fermi energy near the LUMO energy indicates that electrons are the predominant carrier. A Fermi energy near the HOMO energy indicates that holes are the predominant carrier. Accordingly, the Fermi energy is a primary characterizing property of traditional semiconductors and the prototypical PV heterojunction has traditionally been the p-n interface.
In addition to relative free-carrier concentrations, a significant property in organic semiconductors is carrier mobility. Mobility measures the ease with which a charge carrier can move through a conducting material in response to an electric field. As opposed to free carrier concentrations, carrier mobility is determined in large part by intrinsic properties of the organic material such as crystal symmetry and periodicity. Appropriate symmetry and periodicity can produce higher quantum wave function overlap of HOMO levels producing higher hole mobility, or similarly, higher overlap of LUMO levels to produce higher electron mobility. Moreover, the donor or acceptor nature of an organic semiconductor, e.g., 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), may be at odds with the higher carrier mobility. For example, while chemistry arguments suggest a donor, or n-type, character for PTCDA, experiments indicate that hole mobilities exceed electron mobilities by several orders of magnitude so that the hole mobility is a critical factor. The result is that device configuration predictions from donor/acceptor criteria may not be borne out by actual device performance. Thus, in selecting organic materials such as those described herein for photosensitive optoelectronic devices, it has been found that isotype heterojunctions, e.g, p-p, may have rectifying properties as good as traditional p-n type heterojunctions, although true p-n type is generally preferable when possible. Isotype heterojunctions are discussed further below. Due to these unique electronic properties of organic materials, rather than designating them as "p-type" or "n-type", the nomenclature of "hole-transporting-layer" (HTL) or "electron-transporting-layer" (ETL) is frequently used. In this designation scheme, an ETL will preferentially be electron conducting and an HTL will preferentially be hole transporting. The term "rectifying" denotes, inter alia, that an interface has an asymmetric conduction characteristic, i.e., the interface supports electronic charge transport preferably in one direction. Rectification is associated normally with a built-in electric field which occurs at the heterojunction between appropriately selected materials.
The electrodes, or contacts, used in a photosensitive optoelectronic device are an important consideration. In a photosensitive optoelectronic device, it is desirable to allow the maximum amount of ambient electromagnetic radiation from the device exterior to be admitted to the photoconductively active interior region. That is, it is desirable to get the electromagnetic radiation to where it can be converted to electricity by photoconductive absorption. This indicates that at least one of the electrical contacts should be minimally absorbing and minimally reflecting of the incident electromagnetic radiation. That is, the contact should be substantially transparent. When used herein, the terms "electrode" and "contact" refer only to layers that provide a medium for delivering photogenerated power to an external circuit or providing a bias voltage to the device. That is, an electrode, or contact, provides the interface between the photoconductively active regions of an organic photosensitive optoelectronic device and a wire, lead, trace or other means for transporting the charge carriers to or from the external circuit. The term "charge transfer layer" is used herein to refer to layers similar to but different from electrodes in that a charge transfer layer only delivers charge carriers from one subsection of an optoelectronic device to the adjacent subsection. As used herein, a layer of material or a sequence of several layers of different materials is said to be "transparent" when the layer or layers permit at least 50% of the ambient electromagnetic radiation in relevant wavelengths to be transmitted through the layer or layers.
When an electrode or charge transfer layer provides the primary mechanism for photovoltaic charge separation, the device is called a Schottky device as discussed further below.
Electrodes or contacts are usually metals or "metal substitutes". Herein the term "metal" is used to embrace both materials composed of an elementally pure metal, e.g., Mg, and also metal alloys which are materials composed of two or more elementally pure metals, e.g., Mg and Ag together, denoted Mg:Ag. Here, the term "metal substitute" refers to a material that is not a metal within the normal definition, but which has the metal-like properties that are desired in certain appropriate applications. Commonly used metal substitutes for electrodes and charge transfer layers would include wide bandgap semiconductors, for example, transparent conducting oxides such as indium tin oxide (ITO), tin oxide (TO), gallium indium tin oxide (GITO), zinc oxide (ZO) and zinc indium tin oxide (ZITO). In particular, ITO is a highly doped degenerate n+ semiconductor with an optical bandgap of approximately 3.2 eV rendering it transparent to wavelengths greater than approximately 3900 .ANG.. Another suitable metal substitute material is the transparent conductive polymer polyanaline (PANI) and its chemical relatives. Metal substitutes may be further selected from a wide range of non-metallic materials, wherein the term "non-metallic" is meant to embrace a wide range of materials provided that the material is free of metal in its chemically uncombined form. When a metal is present in its chemically uncombined form, either alone or in combination with one or more other metals as an alloy, the metal may alternatively be referred to as being present in its metallic form or as being a "free metal". Thus, the metal substitute electrodes of the present invention may sometimes be referred to by one or more of the inventors of the present invention as "metal-free" wherein the term "metal-free" is expressly meant to embrace a material free of metal in its chemically uncombined form. Free metals typically have a form of metallic bonding that may be thought of as a type of chemical bonding that results from a sea of valence electrons which are free to move in an electronic conduction band throughout the metal lattice. While metal substitutes may contain metal constituents they are "non-metallic" on several bases. They are not pure free-metals nor are they alloys of free-metals. Further, these metal substitutes do not have their Fermi level in a band of conducting states in contrast with true metals. When metals are present in their metallic form, the electronic conduction band tends to provide, among other metallic properties, a high electrical conductivity as well as a high reflectivity for optical radiation. Another characteristic of metallic conductors is the temperature dependence of their conductivity. Metals generally have a high conductivity at room temperature which increases as the temperature is lowered to near absolute zero. Metal substitutes, for example, semiconductors including, inter alia, inorganic, organic, amorphous, or crystalline, generally have conductivities which decrease as their temperature is lowered to near absolute zero.
There are two basic organic photovoltaic device configurations. The first type is the Schottky-type cell with a single species of organic photoconductive material sandwiched between a pair of metal and/or metal substitute contacts. Conventionally, for n-type photoconductors, a high work function metal, e.g., Au, has been used as the Schottky contact, and for p-type photoconductors, a metal with a low work function, e.g., Al, Mg, or In has been used as the Schottky contact. The charge separation desired in a PV device is induced by exciton dissociation in the space-charge region associated with the built-in electric field at the metal/photoconductor interface. Conventionally, such a device requires different metal or metal substitute pair combinations as contacts since use of the same material at both interfaces would ostensibly produce opposing rectifying junctions. If the same material is used for both electrodes it has been generally thought that the fields generated at the photoconductor-electrode interfaces are necessarily equal in magnitude and opposite in direction so that no net photocurrent is generated in the absence of an external applied voltage. While it is possible for charge separation to occur at both interfaces and be additive, it is generally preferable to have all charge separation occurring at one interface. For example, this can be achieved if the non-rectifying interface has little or no barrier to carrier transport, i.e., if it is a relatively low resistance contact. This may also be referred to as an "ohmic" contact. In any event, in photosensitive optoelectronic devices it is generally desirable that the interfaces either contribute to the net charge separating action or present the smallest possible resistance or barrier to carrier transport.
A sample prior art organic Schottky device is shown schematically in FIG. 2A. Contact 2A01 is Ag; organic photoconductive layer 2A02 is PTCDA; and contact 2A03 is ITO. Such a cell was described in an article by N. Karl, A. Bauer, J. Holzaofel, J. Marktanner, M. Mobus, and F. Stolzle, "Efficient Organic Photovoltaic Cells: The Role of Excitonic Light Collection, Exciton Diffusion to Interfaces, Internal Fields for Charge Separation, and High Charge Carrier Mobilities", Molecular Crystals and Liquid Crystals, Vol. 252, pp 243-258, 1994 (hereinafter Karl et al.). Karl et al. noted that while the Ag electrode 2A01 was photovoltaically active in such a cell, the ITO electrode very rarely was photoactive and further that the indications of photovoltaic action at the ITO electrode had poor statistical certainty. Further, one of ordinary skill in the art would expect contact 2A01 not to be transparent.
The second type of photovoltaic device configuration is the organic bilayer cell. In the bilayer cell, charge separation predominantly occurs at the organic heterojunction. The built-in potential is determined by the HOMO-LUMO gap energy difference between the two materials contacting to form the heterojunction. An isotype heterojunction has been discussed in an article by S. R. Forrest, L. Y. Leu, F. F. So, and W. Y. Yoon, "Optical and Electrical Properties of Isotype Crystalline Molecular Organic Heterojunctions", Journal of Applied Physics, Vol. 66, No. 12, 1989 (hereinafter "Forrest, Leu et al.") and in an article by Forrest, S. R., "Ultrathin Organic Films Grown by Organic Molecular Beam Deposition and Related Techniques", Chemical Reviews, Vol. 97, No. 6, 1997 (hereinafter Forrest, Chem. Rev. 1997) both of which are incorporated herein by reference. Forrest, Leu et al. describe two isotype solar cells depicted in FIG. 2B and FIG. 2C. FIG. 2B shows a device consisting of an ITO electrode 2B02 on a substrate 2B01 covered with a layer 2B03 of copper phthalocyanine (CuPc) and a layer 2B04 of PTCDA with a top electrode 2B05 of In. In a second device, with reference to FIG. 2C, an ITO electrode 2C02 is again placed on a substrate 2C01. Then a CuPc layer 2C03 and a 3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (PTCBI) layer 2C04 are placed in order with a Ag electrode 2C05 on top. This prior art had only one transparent electrode and it was on the bottom of the device. It was also noted in this reference that these organic photovoltaic devices suffered from a high series resistance.
As in the case of Schottky devices, unless an interface, at a contact, for example, is contributing to the charge separation, it is desirable that the interface produce the smallest possible obstruction to free current flow. In bilayer devices, since the dominant charge separating region is located near heterojunction, it is desirable for the interfaces at the electrodes to have the smallest possible resistance. In particular, it is known in the art to use thin metal layers as low resistance, or ohmic, electrodes, or contacts. When ohmic contacts are desired, a high work function metal, e.g., Au, has been used as the positive electrode, i.e., anode, in photosensitive optoelectronic devices. Similarly, a low work function metal, e.g., Al, Mg, or In, has been used to make an ohmic negative electrode, i.e., cathode.
Herein, the term "cathode" is used in the following manner. In a PV device under ambient irradiation and connected with a resistive load and with no externally applied voltage, e.g., a solar cell, electrons move to the cathode from the adjacent photoconducting material. With an applied bias voltage, electrons may move from the cathode to the adjacent photoconducting material, or vice versa, depending on the direction and magnitude of the applied voltage. For example, under "forward-bias" a negative bias is applied to the cathode. If the magnitude of the forward-bias equals that of the internally generated potential there will be no net current through the device. If the forward-bias potential exceeds the internal potential in magnitude there will be a current in the opposite direction from the non-biased situation. In this later forward-bias situation, electrons move from the cathode into the adjacent photoconductive organic layer. Under "reverse-bias", a positive bias is applied to the cathode and any electrons which can move do so in the same direction as in the no bias situation. A reverse-biased device generally has little or no current flow until it is irradiated. Similarly, the term "anode" is used herein such that in a solar cell under illumination, holes move to the anode from the adjacent photoconducting material, which is equivalent to electrons moving in the opposite manner. The application of an external voltage to the device structure will alter the flow of the carriers at the anode/photoconductor interface in a complementary fashion to that described for the cathode and in a manner understood by those of ordinary skill in the art. It will be noted that as the terms are used herein anodes and cathodes may be electrodes or charge transfer layers.
Further, as discussed above, in non-Schottky photosensitive optoelectronic devices it is similarly desirable for the electrodes not merely to form ohmic contacts but also to have high optical transparency. Transparency requires both low reflectivity and low absorption. Metals have the desired low resistance contact properties; however, they can produce significant conversion efficiency reductions due to reflection of ambient radiation away from the device. Also, metal electrodes can absorb significant amounts of electromagnetic radiation, especially in thick layers. Therefore, it has been desirable to find low resistance, high transparency electrode materials and structures. In particular, the metal substitute ITO has the desired optical properties. It is also known in the art that ITO functions well as an anode in organic optoelectronic devices. However, it had not been previously thought that ITO or other metal substitutes could make low resistance cathodes for organic optoelectronic devices. Solar cells had been disclosed in which a highly transparent ITO layer may have functioned as a cathode in some cases, but such ITO cathodes were only disclosed as having been prepared by depositing the charge-carrying organic layer onto the ITO layer by Karl et al. and Whitlock, J. B., Panayotatos, P., Sharma, G. D., Cox, M. D., Savers, R. R., and Bird, G. R., "Investigations of Materials and Device Structures for Organic Semiconductor Solar Cells," Optical Eng., Vol. 32, No. 8, 1921-1934 (August 1993), (Whitlock et al).
Prior art PV devices, e.g., FIGS. 2A and 2B, have only utilized non-metallic materials, e.g., indium tin oxide (ITO), as one electrode of the photovoltaic device. The other electrode has traditionally been a non-transparent metallic layer, e.g., aluminum, indium, gold, tin, silver, magnesium, lithium, etc. or their alloys, selected on the basis of work function as discussed above. U.S. Pat. No. 5,703,436 to Forrest, S. R. et al. (hereinafter Forrest '436), incorporated herein by reference, describes a technique for fabricating organic photoemissive devices (TOLEDs: Transparent Organic Light Emitting Diodes) having a transparent cathode deposited onto an organic ETL by depositing a thin metallic layer, e.g., Mg:Ag, onto the organic ETL and then sputter depositing an ITO layer onto the Mg:Ag layer. Such a cathode having the ITO layer sputter deposited onto a Mg:Ag layer is referred to herein as a "composite ITO/Mg:Ag cathode". The composite ITO/Mg:Ag cathode has high transmission as well as low resistance properties.
It is known in the art of inorganic solar cells to stack multiple photovoltaic cells to create an inorganic multisection solar cell with transparent metallic layers. For example, U.S. Pat. No. 4,255,211 to Frass (hereinafter "Frass '211") discloses a stacked cell arrangement. However, the photolithographic techniques used to fabricate inorganic electronic devices are typically inapplicable to production of organic optoelectronic devices. Photolithography generally involves deposition of metallic layers and inorganic semiconductive layers followed by additional steps of masking and etching. The etching steps involve use of strong solvents which can dissolve the relatively fragile organic semiconductor materials that are suitable for organic photovoltaic devices. Therefore, organic photosensitive optoelectronic device fabrication techniques typically avoid this type of liquid etching process in which deposited material is removed after an organic layer has been deposited. Instead, device layers are generally deposited sequentially with techniques such as evaporation or sputtering. Access to electrodes is generally implemented using masking or dry etching during deposition. This constraint presents a challenge to fabrication of a stacked organic optoelectronic device for which electrode access to the intervening layers in the stack is desired. Thus, it is believed that all prior art stacked cells have the individual photovoltaic cells electrically connected internally and only in series.
For inorganic photovoltaic devices, series connection is not particularly disadvantageous. However, due to the high series resistance of the organic photovoltaic devices noted above, a series configuration is undesirable for power applications due to the reduced efficiency. Forrest, Chem. Rev. 1997 reported that high series resistance in organic solar cells leads to space-charge build-up as power levels are raised with increasing incident light intensity. This leads to degradation of the photocurrent, I.sub.max effectively reducing the fill factor and therefore the efficiency. Moreover, what is believed to be the only previously disclosed organic solar cell with more than one photovoltaic subcell was a tandem, i.e., two PV subcells, with the subcells connected in series. See Effect of Thin Gold Interstitial-layer on the Photovoltaic Properties of Tandem Organic Solar Cell, Hiramoto, M.; Suezaki, M.; Yokoyama, M; Chemistry Letters 1990, 327 (hereinafter "Hiramoto"). Referring to FIG. 2D, substrate 2D01 is glass; 2D02 is ITO; 2D03 is Me-PTC (500 .ANG.); 2D04 is H.sub.2 Pc (700 .ANG.); 2D05 is Au (&lt;30 .ANG.); 2D06 is Me-PTC (700 .ANG.); H.sub.2 Pc (700 .ANG.); and 2D07 is Au (200 .ANG.). This device has the subcells electrically connected internally and in series, thus avoiding the problem of devising a means to make external contact to an electrode within the middle of a stack of organic semiconducting material. Hiramoto's organic tandem devices have just two electrodes: one on top and bottom used to make external connections plus charge transfer layer 2D05 which electrically "floats" between the two subcells". Only one of the electrodes, bottom ITO layer 2D02 was transparent. Top Au layer 2D07 was 200 .ANG. thick and therefore non-transparent. Further, for the reasons noted above, series connection is not an optimal configuration in stacked organic photovoltaic devices for high power applications.
A solar cell may be viewed as a photodiode with no applied bias. The internal electric field generates a photocurrent when light is incident on the solar cell and the current drives a resistive load for the extraction of power. On the other hand, a photodetector may be viewed as a diode with no externally applied bias voltage or a finite externally applied bias voltage. When electromagnetic radiation is incident upon a photodetector with a bias, the current increases from its dark value to a value proportional to the number of photogenerated carriers and the increase may be measured with external circuitry. If a photodiode is operated with no applied bias, an external circuit may be used to measure the photogenerated voltage and achieve photodetection. While the same general configuration of electrodes, charge transfer layers and photoconductive layers may be used alternatively as a solar cell or as a photodetector, a configuration optimized for one purpose is generally not optimal for another. For example, photosensitive optoelectronic devices produced as solar cells are designed to convert as much of the available solar spectrum as possible to electricity. Therefore, a broad spectral response over the entire visible spectrum is desirable. On the other hand, a photodetector may be desired which has a photosensitive response over a narrow spectral range or over a range outside the visible spectrum.
Organic PV devices typically have relatively low quantum yield (the ratio of photons absorbed to carrier pairs generated, or electromagnetic radiation to electricity conversion efficiency), being on the order of 1% or less. This is in part thought to be due to the second order nature of the intrinsic photoconductive process, that is, carrier generation requires exciton generation, diffusion and ionization, as described above. In order to increase these yields, materials and device configurations are desirable which can enhance the quantum yield and, therefore, the power conversion efficiency.
Forrest Chem. Rev. 1997 and Arbour, C., Armstrong, N. R., Brina, R., Collins, G., Danziger, J.-P., Lee, P., Nebesny, K. W., Pankow, J., Waite, S., "Surace Chemistries and Photoelectrochemistries of Thin Film Molecular Semiconductor Materials", Molecular Crystals and Liquid Crystals, 1990, 183, 307, (hereinafter Arbour et al.), incorporated herein by reference in its entirety, disclose that alternating thin multilayer stacks of similar type photoconductors could be used to enhance photogenerated carrier collection efficiency over that using a single layer structure. Further, these sources describe multiple quantum well (MQW) structures in which quantum size effects occur when the layer thicknesses become comparable to the exciton dimensions.